Whenever I try to copy the digit 1 from a PDF file onto another file (Notepad, Word, etc.), it pastes the character . All other digits, for some reason, copy and paste normally. Contacted support but they were unable to assist. I'm using Adobe Acrobat DC. Any suggestions?

Thanks for the info. As you mentioned, its a particular kind of file from which you are facing issue with, you may try using the PDF Fixup Profile "Embed Fonts" in the Preflight tool to embed the font and check if that helps. However, the font does need to be installed on your system.